Apertured screw receiving deformity in sheet metal



April 1953 c. w. COLLINS EI'AL 2,831,388

APERTURED SCREW RECEIVING DEFORMITY IN SHEET METAL Filed Nov. 23, 1953 3654M [0y It? [M1175 11 127 6. 5/727 APERTURED SCREW DEFGRMITY 5N METAL Coy W. Coliins and Erwin G. Zahn, Toledo, Qhio, assignors to Prestole ilorporation, 'ioiedo, @hio, a corporation of Michigan Application Novemb r 23, ii -5S3, Seriai No. 393,792

2 Claims. (Cl. 35-32) This invention relates to apertured impressions or proturberances in sheet metal, adapted to have sheet metal screws threadedly received in their apertures.

When a sheet metal screw is to be threadedly accommodated in a hole in a piece of sheet metal, the edge of the aperture should have a helical configuration corresponding to the pitch of the screw. Obviously this configuration cannot be achieved unless the sheet metal is deformed out of flatness in the neighborhood of the aperture. One such type of deformity which is commonly used is a substantially conical impression or protuberance, bumped out of the metal concentrically with the aperture. A radial slit in the metal of the impression, extending from the aperture, permits the edge of the aperture at one side of the slit to be raised above that at the other side of the slit to provide the necessary helical disposition of the edge of the aperture, the axial distance be-:

tween the edges of the slit at the edge of the aperture being of course equal to the pitch of the screw to be accommodated.

The purpose and object of this invention is to provide a protuberance of this character which combines the advantages of a round aperture, or hole, with the struc tural strength of a well proportioned frustrum of a cone.

The simplest and most expeditious way of producing such apertured impressions or protuberances is by means or" a die which provides a so-called one-shot operation and which has its punch portion and impression-forming portion fixed with respect to one another. The male die member for such a die set has a central cylindrical punch portion projecting axially beyond an encircling substantially conical impression-producing portion, and the female die member has a central here or well which receives the punch portion of the male die member and a counterbore-like conical well which receives the impression-producing portion of the male member.

Although the one-shot die just described is simple and inexpensive to produce and use, it presents a problem which, heretofore, has i not been satisfactorily solved. With this one-shot method the metal is deformed after the central aperture is punched out,' and hence the aperture the metal of the impression is bent out of the plane of the sheet metal on a radius of gyration which represents the 2,83 1,3 88 Patented Apr. 22, 1958 around the edge of the aperture, with the result thatthe aperture would be so distorted out of roundness that it would have an extremely poor fit with the screw to be received therein, if indeed the screw could even be accommodated.

This ditficulty is discussed in the Kost Patent No. 2,169,182; but the patentees solution'to the problem was at the expense of structural strength. He displaced the metal of the protuberance to gain a roun'dhole, but in so doing, the wallof the protuberance was given a gradually varying concavity. The result was a structure considerably-weaker than one having a truly conical formation.

Apparently, in an effort to improve upon the fastener of his Patent No. 2,169,182; the patentee 'Kost devised the multiple operation method of his later Patent No. 2,383,133, whereby the protuberance is first formed and then pierced; but even this expedient did not yield the results sought. its multiple operations increased the cost, and, in some forms thereof, it did not avoid the concavity in the wall of the protuberance; while, in the other, it failed to produce the desired round hole. c

It has now been discovered that a round hole can be produced along with a protuberance shaped for maximum strength, by means of a one-shot die,'if the slant height of the protuberance, i. e. the distance from the edge of the hole to the base of the protuberance increases uniformly from a minimum at the lowest point' on the edge of the hole to maximum at the highest point on the'edge of the hole, providing the slope of the wall ismaintained at a substantially constant angle all around the protuberance. With this helicoidal shape, the base of the protuberance defines a single spiral which begins and ends is enlarged during the subsequent formation of the conical.

angle of the conical surface of the impression, the horiy tontally projected dimension of the metal of the impression will be more or less decreased at any given radial around the rim of the aperture, enlarging the aperture correspondingly at that radial. This enlargement of the aperture in the formation of the impression is referred to as drawback.

Because the angle at which the impression is bent up- ,wardly out of the plane of the metal would vary from .radial to radial around the impression in the case of a strictly conical impression having a perfectly circular base, the drawback would likewise vary from point to point line 3-3 in Figure 1; 7O

at the slit.

However, even after it was discovered that thejdesired results could be achieved with such a helicoidal impression or protuberance, a practical problem still remained, for no method was known for laying out the die so that the correct shape could always be obtained. The die maker had to exercise his judgment as best he could to produce a die which would come close to accomplishing the desired result and then, by filing or otherwise working the die or the master from which the die was produced, gradually evolve a satisfactory tool.

.To overcome this objection, the present invention also provides a simple method for laying out a die for the one-shot production of apertured impressions of the character described, which die produces an impression having a uniform drawback at all radials around the same, so that the aperture is circular and will, therefore, nicely fit a sheet metal screw. Y

With the above and other objects in view, which will appear as the description proceeds, this invention resides in the novel product substantially as hereinafter described and more particularly defined by the appended claims, it being understood that such changes in the precise embodiment ofthe hereindisclosed invention may be made as come within the scope of the claims.

The accompanying drawing illustrates one complete example of the pyhsical embodiment of the invention practi'ced and constructed according to the best mode so far devised for the practical application of the principles thereof, and in which:

Figure 1 is a plan view of a piece of sheet metal having an apertured impression formed therein according to the present invention;

Figure 2 is a side elevational view of the piece of sheet metal shown in Figure 1;

Figure 3 is a sectional view taken on the plane of the Figure 4 is a diagram showing the application of the invention to an apertured impression; and Figure 5 is a perspective view of male and female die mam 1 sheet metal having a substantially flat surface 6. A substantially conical impressionor protuberance 7, deformed out of the metal, projects to one side of the flat surface of the member and has a substantially central aperture 8, the diameter of which is substantially equal to the root diameter of -a sheet metal screw (not shown) to be threadedly received therein.

As may be best seen in Figure 3, the metal of the impression is bent upwardly at an obtuse angle to the flat surface of the piece of sheet metal at all radials around the aperture, and the profile of any radial element of the impression is straight, rather than curved. The purpose of the impression is to dispose the edge of the aperture 8 on a helix so that said edge will be nicely accommodated to the pitch of a sheet metal screw to be received in the aperture, and to this end a slit 10 extends radially from the aperture and one slit-defining edge 12 of the metal has its inner end higher than the other; or, stated another way, the metal at one side of the slit is bent outwardly from the plane of the flat surface on a radius of gyration greater than that of the metal .on the other side of the slit.

Since the metal has no appreciable stretch during the formation of the impression, and since the aperture is formed before the impression or protuberance is bumped out of the surface of the metal, each point on the edge of the aperture moves upwardly and outwardly in an arc designated by 14 (see Figure 4) as the metal is bent about its axis of gyration, and the diameter of the aperture at any given point is thus increased in proportion to the horizontal projected distance through which that point on p the edge moves during deformation of the metal to provide the impression.

If the impression or protuberance were strictly conical the aperture.

A die set for the one-shot production of an apertured impression of the type just described is shown in Figure 5. The male die member, designated generally by the numeral 16, has a central cylindrical punch portion 18 which is fixed with respect to the remainder of the die member and which projects axially beyond an encircling impression-forming portion 19. The impression-forming portion is of course helicoidal and has an abrupt step 20 which forms the slit in the finished impression. Surrounding the helicoidal portion of the die member is a flat base portion 21, and the helicoidal portion tapers upwardly from this base portion to the cylindrical punch portion.

The female die member, designated generally by 23, has a central bore or well 24 in which the punch portion 'of the male die member is received, surrounded by a counterbore-like helicoidal cavity 26 in which the helicoidal impression-forming portion of the male die member is received.

In use, the female die member is held against one surface of a sheet of metal to be provided with an apertured impression, and the male die member is driven toward the female member from the other side of the sheet. Hence the punch portion of the male member pierces the sheet of metal before any substantial deformity thereof occurs, and after the cylindrical punch portion on the male die member has penetrated the sheet to form the aperture, the helicoidal portion of the male die member forces a localized area of the metal into the helicoidal cavity in the female member to produce the impression. Because of drawback, the aperture is enlarged during the production of the impression, permitting free withdrawal of the punch portion from the hole after the impression has been made.

In order to assure that drawback will be uniform at all radials around the aperture, so that the aperture in the finished impression or protuberance will be round, the helicoidal impression or protuberance should have a uniformly increasing slant height and the slope-of its wall should be at a substantially constant angle all the way around. The minimum height of the protuberance, i. e. the vertical distance from the sheet in which the protuberance is formed to the edge of the hole at its lowest point, which experience has shown yields the best results, lies between .020 and .032 inch, depending upon the root diameter of the screw to be accommodated, and for most cases, this dimension may be .025 inch, it being understood that sheet metal screws are not generally made in a great variety of sizes and seldom are ye large in diameter.

The slope of the wall of the protuberance obviously may not be too steep, for otherwise it can not accommodate the screw or, more specifically, the edge of the hole at the top of the protuberance will not properly engage between the threads of the screw. Sixty degrees is about the practical maximum limit for the angle of this slope. Nor should the slope of the protuberance be too flat, since the strength of the protuberance decreases as the slope approaches flatness. Twenty degrees is about the practical minimum limit.

These various requirements, and particularly the uniform straight slope of the protuberance wall and the uni- (that is, if it had a circular base) the drawback would vary widely from point to point around the aperture, due

form draw back all around the edge of the hole are depicted in the diagram, Figure 4, which may be taken to represent the top surface of the metal substantially at the slit 10, and in which the axis of the impression is designated by the numeral 28, the lower slit-defining edge of the metal of the impression is designated by the line 29, and the upper slit-defining edge by the line 30. Referring to this diagram and considering an element of the deformity at any given radial around the aperture:

X =The amount of the drawback of the metal at the element under consideration;

H=the height from the base of the impression to the edge of the aperture at the element under consideration; Y=the slant height, i. e., the radius of gyration of the impression element under consideration;

A=the horizontal distance from the edge of the aperture, as finally drawn, to the center of the radius of gyration of the element; and

In addition to the foregoing factors depicted on the diagram, the following are also pertinent, as will appear hereinafter.

=the element under consideration, expressed in degrees of rotation about the axis of the aperture from the element of lowest height;

R ==the root diameter of the sheet metal screw to be received in the aperture; i. e., the desired diameter of the finished aperture; and

P=the pitch of the screw to be received in the aperture.

To assure uniform results and enable a reliably accurate duplicate of the desired configuration to be made by the toolmaker whenever he is called upon to make a new die, a practical guide, or considered loosely, a formula has been evolved by which the radius of the spiral shaped base may be determined and correlated to the root diameter of the screw. this guide or formula is simply the following equation:

The equation will yield a set of values which correspond to the upper (i. e., the convex) surface of the impression. The female die member cavity should of course be formed to substantially the dimensions given by the calculations, while the male die member should have its conical impression-producing portion formed to the same values but with allowance made for the thickness of the material to be worked.

Through the use of the foregoing equation or, loosely, formula, and with the minimum height of the protuberance ranging from .020 to .032 inch as pointed out hereinbefore, the minimum angle for the slope of the side wall of the protuberance will be found to be about twenty degrees. This gives the protuberance or impression a shape which is not only different from all such previous protuberances, but one which has considerably greater strength.

Essentially, the characteristic differences which the protuberance formed in accordance with this invention possesses over all prior constructions lies in the fact that the side of the protuberance at any radical thereof, i. e. any radial plane thereof is a part of a truncated cone and not concave or convex: that the slant height (Y in Figure 4) increases uniformly from a minimum at the lowest point on the edge of the hole to a maximum at the highest point on the edge of the hole; and that the slope of the wall is maintained at a constant, uniform angle, not substantially less than twenty degrees and not substantially more than sixty degrees, all around the protuberance so that the base of the protuberance defines one true and complete spiral.

From the foregoing description taken together with the accompanying drawing it will be apparent that the present invention provides an apertured impression in sheet metal for threadedly receiving a sheet metal screw, produced by a one-shot operation and having a substantially uniform drawback around the aperture, so that the hole is substantially perfectly round, and the invention also enables the production of dies for forming such impressions without the need for cut-and-try experimentation.

What we claim as our invention is:

1. A fastener of the character described comprising: a sheet metal member; a generally conical protuberance formed in the member and projecting from one face thereof, the protuberance having a round hole in its apex portion; the wall of the protuberance at one side thereof having a slit which extends radially from the edge of the hole to the base of the protuberance, the edge portions of said slit being divergent toward the hole and spaced apart axially at their junctions with the edge of the hole, and the edge of the hole being helical to have threaded engagement with a screw to be secured in the hole; and said protuberance being characterized by the fact that its wall thickness is on the order of onehalf the pitch of the helix defined by the edge of the hole and hence substantially one half the pitch of the screw to be accommodated and that its wall has a uniform straight slope from substantially the edge of the hole down to the base of the protuberance with the angle of the slope substantially constant all the way around the protuberance and lying between approximately twenty degrees and sixty degrees; by the fact that the slant height of the protuberance increases uniformly from a minimum at the lowest point on the edge of the hole to maximum at the highest point on the edge of the hole; and further by the fact that the base of the protuberance defines one continuous spiral beginning and ending at the slit.

2. The fastener of claim 1 further characterized by the fact that the radial distance from the axis of the round hole to any point on the spiral base of the protuberance is substantially equal to o: (36O) +Rd where P is the pitch of the helix defined by the edge of the hole and hence substantially equal to the pitch of the screw to be accommodated, on is the point on the base under consideration expressed in degrees of rotation about the axis of the hole from the lowest point on the edge thereof, and Rd is the root diameter of the screw. 

